Saturday, 29 October 2016

The Hydrogen Steamer: Stealth Spaceship Concept

Further to the Stealth in Space discussion, we found a type of spaceship perfectly suited to moving about the Solar System undetected. It is called the 'Hydrogen Steamer', and relies on using large amounts of liquid hydrogen.

Here is a discussion of the mechanisms involved, the capabilities and potential designs for the Hydrogen Steamer.

Cold StealthStealth in Space relies on making sure that your enemy does not receive any of your electromagnetic emissions.

Some methods of achieving stealth rely on redirecting or obfuscating your emissions. The largest component of these is Infrared radiation: the heat that your spaceship must remove.

The Cosmic Background Radiation. Average temperature: 2.73K

Naturally, the best method of reducing Infrared emissions is by not producing any in the first place. A spacecraft at 3 Kelvin temperature cannot be distinguished from the background radiation in space by any physically possible sensor. Even at a more moderate 22K, it is extremely difficult to detect. This is Cold Stealth.To maintain this level of stealth, the spacecraft cannot use radiators, nor can it leave a trail of bright hot exhaust. How does it work?

Typical space warship. Large, glowing radiators. From Children of a Dead Earth.

Most machines, including the crew, cannot function at such a low temperature, barely above absolute zero. They have to be kept at much higher temperatures. The crew quarters, for example, are kept at a cozy 300K and high-energy equipment such as lasers might rise to to 700K or above.The obvious solution is to only cool the external surface of the spaceship to cryogenic temperatures, and keep the interior at regular temperatures. This precludes the use of radiators, so another method of dealing with waste heat has to be used. Even if nothing is running on-board, the spacecraft still absorbs sunlight.Conventional rocket engines leave an exhaust trail that starts out at 3000K temperatures, then decreases as it expands into vacuum. However, due to how much propellant is released per second, the expansion is slow enough to create a massive ball of hot gas behind the nozzle. This makes the exhaust very easy to spot. A spaceship relying on cold stealth has to use another form of propulsion.

There are three parts to solving these problems:-A cryogenic heat sink-An insulating hull-A stealthy propulsion systemThe heatsinkIn conventional SF, heatsinks were considered as a short term solution to the high waste heat outputs of weaponry. They reached elevated temperatures to extract the maximum amount of waste heat per unit of coolant.

Glowing heatsinks in Elite:Dangerous

All waste heat was eventually removed by radiators. These are run as hot as possible to minimize their surface area and the mass dedicated to them. For a stealth spaceship, the objectives are different. A cryogenic liquid heatsink is boiled off to remove a minimal amount of waste heat. The resulting gas is fed to the propulsion system. The only two candidates for this are liquid hydrogen and liquid helium.

The temperature/tesla curve for various superconducting materials.

Both boil at temperatures very near absolute zero, but helium boils at 4K while Helium stays liquid up to 22K. Helium can be used to directly cool the spaceship to background temperature, but hydrogen has twenty times the latent heat of vaporization (the energy required to move it from liquid to gaseous form). This means that we'd need twenty times less hydrogen per second to stay cool, and the difference between 4K and 22K at long range should be negligible. The choice between the two depends on the setting your are building. In a sensor-poor environment, hydrogen offers much better performance and higher endurance per kilo. Helium is 76% denser and very conductive to heat in its superfluid state, simplifying design. Helium might be the only solution to a sensor-rich setting, where short ranges or large numbers of sensitive satellites can detect even 22K temperatures.If hydrogen is selected, a 1kW source of waste heat will require 2.24 grams of liquid hydrogen per second, or 8kg per hour, to cool down to 22K. If the spaceship is discovered or decides to end its stealth mode, it can further heat the hydrogen gas. This removes 14kJ of heat per kg for every Kelvin above 22K. If it is used to directly cool the crew quarters, and allowed to reach 300K before it is dumped overboard, one kilo of hydrogen will take away 3.94MJ of waste heat with it. If it used to cool a 700K piece of equipment (about 430 degrees Celsius), it can take away more than 13MJ per kilo. The insulating hullThe cryogenic heat sink is of no use if hot spots on your spaceship leak Infrared radiation and reveal your position. Constructing a spaceship that can uniformly cool its exterior is hard, which is why the simplest designs uses nestled shells.

Like a submarine, the stealth spaceship will have a very cold exterior shell, with liquid hydrogen running along pipes on its inner surface, and a 'hot' pressure vessel inside that holds equipment and crew. The gap between the shells is used to evacuate the hydrogen gasses produced. The exterior surface of the outer shell is of particular concern. It is what the enemy 'sees'. A regular metal surface, even if cooled to cryogenic temperatures, is quite reflective. RADAR and LIDAR will bounce off it and produce a strong return signal. Sunlight will turn it into a bright beacon.

Aluminium emissivity: 0.03. Vantablack emissivity: 0.999+

The solution is VantaBlack. It is one of the products of research into the optical properties of carbon nanotubes. It can absorb 99.9%+ of all incident light. Sunlight, the biggest problem, will be completely absorbed, and no reflected light will reach enemy sensors. Advances in this technology can extend this characteristic to radio waves. If the exterior surface is coated with this material, it can become 'blacker than black' across most of the electromagnetic spectrum.The overall shape of the outer shell is important too. The stealth spaceship will want to minimize how much energy it receives, minimize the amount of reflected energy, and reliably contain liquid hydrogen for a long period of time.A shape which corresponds to these requirements is a very thin cylinder with an opening for nozzles in the middle. The rounded shape disperses reflected signals across a much wider area than flat sides. A cylinder is the second-best shape for containing cryogenic fuels after a sphere, but a sphere would absorb much more sunlight than a thin cylinder pointed end-on to the Sun. As the position relative to the Sun has to be kept fixed, the engine must be able to swivel around the centre of gravity to allow it to accelerate along different axis. To allow acceleration along multiple axis, a swivelling nozzle is kept in the middle of two long tanks of propellant. The propulsion systemThere is no point in flinging a cold shape into space if it has no way to move afterwards.If the enemy detects the initial boost, they can calculate the trajectory for months in advance. If the enemy's forces change position, the stealth spaceship will miss them entirely. If a stealth-hunter is sent out to find it, it cannot evade.

Highly visible, very large trail of a rocket in upper atmosphere.

The primary requirement of a stealth propulsion system is that it does not shoot hot gas into space. A secondary requirement is that it does not consume a lot of electricity. Producing electricity creates waste heat, and that heat has to be removed by boiling liquid hydrogen, in addition to what is absorbed from the Sun.The solution comes in the form of a solar-thermal pulsed rocket. This rocket engines takes in sunlight into a spherical solar furnace with a heating element.

A small amount of sunlight escapes through the opening into the furnace. Most of it eventually heats up the heating element to a very high temperature (3000K+). Tungsten is a suitable material for this element. Hydrogen propellant is injected into the chamber in bursts. It heats up, and pressure in the chamber increases. A shutter to the nozzle releases the hydrogen at high velocity. A de Laval nozzle allows the propellant to expand before it leaves the engine. Using perfect gas laws (pV=nRT), dropping the propellant from 3000K to 20K temperature requires a decrease in volume of a factor 150. A rocket nozzle exchanges pressure and temperature for increased volume of the propellant. In a confined space, this means the propellant will shoot out of the nozzle. The volume increase necessary for the temperature drop in a ball of hydrogen squirted at the top of the nozzle can be approximated by the ratio of diameters between the top and the nozzle opening. A 150x reduction in volume entails a 5.3x increase in diameter. Therefore, the nozzle must be at least 5.3 times wider at the opening than at the throat.The propellant used is hydrogen gas, boiled from the liquid reserve by the cryogenic cooling system. Pulsed operation allows for the hydrogen to reach temperatures very near that of the heating element, maximizing efficiency. Exhaust velocity can reach 8km/s, and it can generate 0.34N of thrust per square meter exposed to the Sun. Performance varies depending on where the ship is in the Solar System. A large Zone Plate made up of supercooled VantaBlack can be deployed in front of the spaceship to collect more sunlight, at the cost of absorbing between 25 and 50% of the incoming sunlight. For increased stealth, the inlet uses cooled optics to direct the sunlight onto a lens, which focuses it through a pinprick hole into the furnace. Sunlight reflected from the inside of the furnace can only come out through this hole. It will create a new narrow cone of light going from the spaceship to the Sun. Coincidentally, the hardest way to detect a spaceship is to have the Sun at your back and only a pinprick of light to pick up. The cone of light could compromise the spaceship, but it would be very difficult to do so. For maximum stealth, the sunlight inlet can be pulsed. A shutter opens, allowing light into the furnace. It closes before light can bounce back out.

LCD shutter

Mechanical shutters are unable to spin or move quickly enough to be useful. A 300m distance between inlet and furnace would require a spinning circular shutter to reach 180000000 degrees per second (30 million RPM). LCD shutters, with transitions between opaque and transparent measured in nanoseconds, must be used. A shutter time of 50ns allows for inlets to be as short as 15 meters. The shutter material would absorb half the sunlight, so it has to be supercooled by liquid hydrogen so that it does not emit infrared radiation. This would halve the overall propulsion system's efficiency, but allows for extreme endurance.A spaceship equipped with such a propulsion system will have nearly all the sunlight touching it going into the solar furnace. The heat capacity we use is that of hydrogen at 3000K. This is possible because the sun's surface is at 6000K, and we are operating on the same principles as a looking glass focusing sunlight on an ant. The hydrogen is boiled by waste heat from several sources, such as unavoidable sunlight, a shutter system, crew heat or the furnace's imperfections. We do not break the laws of thermodynamics, as the hydrogen is moving the waste heat, not eliminating it. Essentially, the engine is a heat pump: it concentrates the absorbed sunlight into a point, and cools off that point with cold hydrogen. At the nozzle, the hydrogen absorbs 60MJ/kg or more (heat capacity rises with temperature, from 14kJ/kg/K at 100K to 20kJ/kg/K at 700K and so on).

A designHere, we will design a stealth spaceships to work out the capabilities and uses it might have. The mission is to travel from Mars to Earth, and stay there. It will depart from Mars on top of a conventional booster, which will impart 2.94km/s. It then follows a Hohmann trajectory to Earth.The trip duration is 260 days. The deltaV requirement is 2.65km/s for the insertion burn, and another 3.5km/s for manoeuvres (enough to drop from geostationary to low orbit for an attack run). It is required to stay around Earth for 2 years, enough time for a replacement to be sent at the end of the Earth-Mars synodic period. Between Earth and Mars, sunlight averages 980W/m^2. Around Earth, it is 1370W/m^2. This design does not use a shutter system, so when the spacecraft is accelerating, all sources of waste heat are dumped overboard as propellant. We want the spaceship to carry 1000 tons of useful payload to Earth. The liquid hydrogen tanks carry ten times their mass in propellant. A 1mm thick VantaBlack external shell masses about 2.3kg/m^2. Energy generation relies on a 30% efficient nuclear reactor paired to an MHD generator. It masses 1 ton and can produces 1MW. Most of the time, it is powered down to the minimum level required by on-board systems. This can be as low as 1kW, with 2kW of corresponding waste heat.The deltaV requirement translates into a mass ratio of 2.15. This works out as a total mass of 2150 tons. 115 tons of the dry mass is now considered to be the hydrogen tanks. If the payload has a density of 1200kg/m^3, like in a submarine, it fits inside a cylinder 3m wide and 117m long. Liquid hydrogen has a density of 70.8kg/m^3, so the propellant fits inside two cylinders 3m wide and 1148 meters long. Total length is 2.4km. The VantaBlack cover takes 52 tons out of the payload.

The final shape is needle-like, with a width-to-length ratio of 800. However, with most of the mass concentrated in the centre, the spacecraft can turn around without difficulty. This is important when it comes to maintaining the nose pointed at the Sun. With swivelling pairs of nozzle in the middle, it does not have to turn to accelerate in any direction.

The nose is 3m wide and has a surface area of 7.07m^2. Its entire surface is an inlet for the solar thermal pulse rocket. Without a deployable Zone Plate, such as during the transit between Mars and Earth, the engine only produces 2.4 Newtons of thrust. It consumes barely 0.16 grams of liquid hydrogen per second. Over the course of 260 days, it expends 3.64 tons, dropping deltaV by 11m/s.Around Earth orbit, it deploys a 100m wide Zone Plate. This lens focuses sunlight into the inlet. It can be very lightweight if inflatable technology is used. This powers the engine with 5.35MW of solar energy, allowing the stealth spaceship to produce up to 1.35kN of thrust.Initial acceleration is 0.625mm/s^2. It rises to 1.34mm/s^2 when the tanks are emptied. At an average acceleration of 0.985mm/s^2, it can burn through its deltaV reserve in 72 months.Without the deployable Zone Plate, it can run the engine for as long as 161 years...An alternative method of propulsion is described here: expansion-cooled nuclear thermal rockets could allow Hydrogen Steamers to do away with solar power and accelerate around the solar system at accelerations of several meters per second squared.

What can it do? What are the consequences?Once a solar-powered Hydrogen Steamer has inserted itself into Earth orbit, the stealth warship has hide several hundred tons worth of ammunition for years, decades or more if it has no reason to move. It it needs to change an orbit, it can add 170m/s per day to its velocity, and do so for a year and a half. The ammunition can be a massive amount of shrapnel to wipe out an orbit through the Kessler Syndrome, a fleet of missiles to devastate an enemy fleet before it even sorties, or a large laser to back-stab targets and slip away again. On shorter missions, it can handle a crew without any significant increase in the amount of liquid hydrogen expended. It even serves as the perfect platform to mount a telescope on and detect other stealth ships. In terms of military tactics, introducing stealth ships is the equivalent of punching a hornet's nest. The standard fare of bright, bold warships pumping out gigawatts without care, streaking across the Solar System laden with weapons, are forced to become meek and paranoid affairs, as a stealth ship can dump a thousand tons of weapons out of nowhere, at any time.From a political standpoint, stealth ships, especially the long-endurance sort equipped with nuclear warheads, as a nightmare several times worse than modern-day ballistic submarines prowling the seas. Any declaration of war will mean near-immediate retaliation by the enemy's stealth ships already in position, unseen, over your head. Civilian infrastructure, unprotected and unable to escape even the 170m/s per day of the example design above, are utterly at the mercy of such ships. Peacetime militaries will dedicate an improbably large amount of time and resources into trying to detect and track the stealth ships. They cannot start hostilities without being confident that they can all be shot down in short order, and losing track of even one of these ships will cause a panic in intelligence agencies...Militaries, once wars are well underway, will want to preserve the energetic and expensive warships from a sudden attack. They will keep their spaceships stationed in hard-to-reach orbits, that require deltaV levels inaccessible to the solar-thermal pulse rocket. When moving or repositioning, they try to continuously accelerate so that stealth ships cannot catch up. Combat will preferrably be done further away from planets than necessary, even in interplanetary space, to avoid the stealth ships each side supposedly put into place years ago. From an economic perspective, stealth ships are an excellent tool for small militaries. Just like Malaysia and Chile purchasing the Scorpene submarine, it allows countries to scare away much larger fleets at lower cost. Larger countries could start a business selling them to anyone who wanted to protect themselves 'cheaply'.On the other hand, stealth ships could carry a burdensome stigma. A stealth ship in orbit around your planet usually means someone is willing to drop missiles and nukes on you, if not today, but years, even decades from now. It might be a hostile enough act to trigger war...Depending on the setting, stealth ships can have a larger or smaller role in tactical warfare, alongside their 'hot' brethren. Stealth ships can be used as an extra layer to regular warships: they would enter orbit as cold, undetectable objects, position themselves advantageously, then unfold the radiators and strike. If the energy levels are low enough, and it has enough liquid hydrogen to cool itself off, it might even escape detection after firing and re-position for another strike. However, if the regular spaceship has a multi-gigawatt reactor and several meters per second squared acceleration, then the equipment required to achieve cold stealth becomes a hindrance rather than an advantage. Stealth ships would be relegated to a more immobile role, away from the battlefields, unable to both keep up with the fleet and stay cold. Further enhancements-IR filterSunlight is actually a wide spectrum of wavelengths. Most of it concentrated between 400 and 600nm wavelength. Placing an infrared filter between the inlet and the furnace will allow most of the sunlight to go in, but the returning radiation will be absorbed.

At 3000K, the pulse engine's furnace radiates in between 750 and 1500nm (Infrared). This would remove the requirement to have shutters, and also eliminate the cone of light a 'naked' engine would bounce back to the Sun.

Before the exhaust expands and cools down, it is very hot. It radiates strongly in the infrared, and is very visible. If the exhaust nozzle was straight, the hydrogen would shine brightly before expanding. With a pulse engine, it would appear to the enemy as a rapid series of bright flashes: easy to pin down and detect.A serpentine nozzle obscures the hydrogen while it cools behind a bend. It is already used on aircraft today to reduce their thermal signature. Note: Isaac Kuodeveloped the concept from a suggestion on the Stealth In Space Is Possible series of posts. This post contains contributions from his work, and the Solar Thermal Pulse engine presented here is my understanding on a design he came up with alone. Credit is due to him.

78 comments:

Might using stealthed nuclear warheads to illuminate such craft be possible? Position them far from a formation and place enough sensors to spot an occlusion. Detonate enough for mark their vector and then hound them with shining radioactive 'paintballs' to mark them for observation by telescopes/sensors. Then eliminate them.

I'm aware that this detection idea probably WILL NOT work, but I am interested to know how it won't. Will it not work due to a matter of scale '(too much space, not enough warheads') or a more technical matter? Or both?

Oh it will work, but it is wasteful as you described it. If you have enough sensors to spot occlusion, you can just use the background starlight.

What you touched upon however is fluorescent detection. If you pump X-rays into space, either with a massive Free Electron laser or a 'naked' nuclear bomb (no filling to convert Xrays into heat), then you can activate the target material in a way that it starts radiating X-rays itself.

The number of photons released will be very low, but it cannot be countered by any method.

While I find the concept intriguing, I am still of the opinion that the delta between the spaceship hull radiating at 22K and the background environment of 3K is simply going to be too much for any "stealth" ship to be able to escape detection very long.

The other issue which you touched on is the enormous difference in performance between your "stealth ship" and any contemporary military or even civilian craft. The best analogy I can think of is a WWI era U Boat being actively hunted by a pack of modern destroyers. When submerged, the U boat can make @ 8 knots, while the turbine powered destroyers are repositioning at @ 30 knots, and sending helicopters capable of moving even faster. (Even using sub vs sub analogies isn't better, your WWI era U boat is being hunted down by the USS Virginia....)

Controlling reflectivity and emissivity are going to be huge concerns, but I'm not sure even something like VantaBlack is going to be the cure all. The ship is being "illuminated" by everything from radio waves to high energy cosmic radiation, so there are going to be wavelengths that the coating is not going to be fully absorptive of. Even a full coating of metamaterials will only work to refract so many wavelength bands around the ship. By analogy, a modern submarine could be coated with a metamaterial optimized to refract sonar around itself, leaving it invisible underwater, but it will still be visible to surface lookouts if it is surfaced or running just below the surface (i.e. snorkelling or raising communications masts and periscopes). Space navies will be, or become quite adept at looking for things in unusual or overlooked "windows" in the spectrum.

You might be right. However, cooling down the spacecraft's external shell from 22K to 3K requires 7.4 times the liquid hydrogen and a power output of 7.4 times the waste heat being moved.

A 50% efficient solar panel can provide the power at the cost of halving the thrust and overall increasing the liquid hydrogen consumption by a factor 13.

What will this do? It will reduce the stealth spaceship's endurance from 161 years to 12.4 years. That's still miles better than modern submarines, and we know how much of a threat they are.

As for the difference in a maneuverability, I think it is a good thing. if stealth ships had both the ability to be undetectable (and the 3K heat-pumped version is physically impossible to detect) and the ability to follow fleets, then space navies will be composed of nothing but stealth spaceships. Monocultures are boring!

The VantaBlack is a bundle of carbon nanotubes that trap incoming photons, instead of refracting them. The electron properties of carbon allow this to happen. Extremely long or short wavelengths might require non-carbon additions to the material, but I think it is possible to get a good coverage of the spectrum the spaceship is likely to show up in.

Remember, as discussed in the final part of Stealth in Space, getting anomalous readings from a corner of your telescope's CCD is not enough to confirm that it is a spacecraft. Getting the same reading repeatedly, in different locations, and establishing that it is hostile, is still not enough for you to do anything about it.

Blackbody emissions require that a sensor platform be cooled below the temperature of a target source, otherwise the platform's own blackbody radiation will flood the sensors at the wavelengths it is trying to detect. Thus, the 22K shell will only be detectable if the sensor platform were cooled to about 20K. Also,you need to think about the photon flux. At IR wavelengths, it would be virtually impossible to detect the 22K photon flux at greater than 50 000 km, unless the collector dish were HUGE (1 000 000 m^2, for example... this will give you about 1 500 000 km range).

As for the reflection of EM radiation from varied sources: the amount of energy "lighting up" the ship from such sources would be microscopic, at best (the sun is BY FAR the most powerful source), and would produce negligible "return" emissions at any distance. Furthermore, you would not know the source of the original signal, and would therefor have a difficult time resolving the reflected signal from all the natural background sources.

What you present is a means of making space navies expand and extend their search envelopes, and present a means of bringing something like unrestricted submarine warfare into the political, military and economic calculations. If a "stealth" ship were to be discovered in Earth's Hill Sphere, the political reaction would be to treat it like a declaration of war. About the only place something like that would not be an automatic trigger is if it was launched and in a defensive orbit around its own home planet, but any inbound constellations would have weeks or months of observation time to attempt to find them (and if I'm coming, you already know this, so using "active" means to illuminate the battlespace and incorporating sensors from Earth's orbit, the constellation and any other means possible to create massive virtual telescopes will be the order of the day).

So my view is a stealth ship is going to be a diversion of resources for the owning navy, but capable of causing either a political and military crisis if it is dispatched to another planet, or leaving your own planet exposed to fairly extreme measures by the enemy military attempting to mitigate the threat. Ultimately, I may decide to send torch missiles accelerating at 100g at your planet or asteroid rather than risk an expensive constellation. Getting an gigaton force impact will spoil your day: http://www.nextbigfuture.com/2009/02/unmanned-sprint-start-for-nuclear-orion.html

"High-G Asteroid Interceptor

An unmanned Orion asteroid interceptor was designed. It would not need shock absorbers. Artillery arming, fusing, firing system for shells are regularly built to take 1000 Gs.

There was a three page paper: Nuclear explosive propelled Interceptor for deflecting objects on collision course with Earth. Johndale Solem, Los Alamos, proposed unmanned vehicle. No shock absorber or shielding. The pulse units were 25kg bombs of 2.5 kiloton yield.

Get to high velocities with only a few explosives and small shock absorbers or no shocks at all. Launch against a 100 meter chondritic asteroid coming at 25 km/sec. 1000 megatons if it hits. Launch when it is 15 million kilometers away and try to cause 10000km deflection. A minimal Orion weighing 3.3 tons with no warhead would do the job. 115 charges with a total of 288 kiloton yield. Launch to intercept in 5 hours. Ample time to launch a second if the first failed."

"If a "stealth" ship were to be discovered in Earth's Hill Sphere, the political reaction would be to treat it like a declaration of war."

Ah, but this is where a good author distinguishes himself from the rest. A good author would look back to historical examples of nuclear submarines being detected near foreign shores and count the number of times war was declared over it. Here's something recent: http://www.dailymail.co.uk/news/article-2188791/Russian-attack-submarine-slipped-past-US-Navy-patrolled-Gulf-Mexico-weeks-undetected.html

Not everything is black and white, not every state is a paranoid reactionary warmonger and some wars can be won and lost but printing out your sky scan and sending to the enemy the location of all their stealth ships.

Detecting stealth ships orbiting another planet is very difficult, I believe. Against the very warm, very wide disk of the planet, stealth ships would have to heat up so that they do not look like a cold spot against a hot background!

A stealth ship's tactical usefulness is similar to that of a hidden minefield around the enemy's port. You declare war, watch their spaceships depart from low orbit and laugh as they are wiped out by close range nuclear blasts coming out from nowhere. No point defense, no defensive formations, no counter-stealth tactics. Depending on the technology level, it can be simple behind-the-line observation of warships trying to angle their radiators, or a fleet of combat mirrors suddenly redirecting an interplanetary laser beam up the exhaust pipes of unwitting targets...

Torch missiles at 100G? Two problems.-If you can launch them at 100G, I can do so as well, otherwise we'd be signing concessions, not going to war. Then it devolves to an interplanetary shooting fest.-At 100G, it takes 6 hours on average to get to Mars. That's 6 hours to get interceptors into orbit and catch the target with puffs of gas. At 22000km/s, that's all you need to get the attacking missiles to disappear.

Redirecting an asteroid is a poorer choice. Even if the missiles stays hidden during acceleration, the impact will be very visible. A block of stone in the sky coming at you with several week's notice is a not very effective if both sides can put out gigaton impactors on short notice.

Since what you are describing can only be considered an existential threat to any spacefaring power, I am not quite as sanguine about how they will view such things being discovered in *their* Hill Sphere. Responses to asymmetric threats like this could also cover a huge range of possibilities, as Chinese "Unrestricted Warfare" doctrine or Russian "Hybrid Warfare" doctrine suggest.

EarthForce might not nuke the offending spacecraft if their cyberwar or economic warfare units are creating havoc with the Martian Stock exchange or banking system. The event might even be as simple as a sudden currency revaluation (much like China has done multiple times with their currency) or "speculation" on futures markets, things which might not even be apparent as "attacks" at first (and if EarthForce is using PSYOPS to spread rumours in the markets, the actual trigger pullers will be Martian investors themselves).

You point out another difficulty in the idea of a stealth ship: it needs to blend into the background regardless of where it is viewed from. If it is inside the Hill Sphere of a planet (either in the offensive role or defending the home planet), then you now have the added complication that it needs to be "cold" to be invisible for ground and space based sensors from certain angles, but "warm" for anyone holding the Gravity gauge in the High guard position.

As for 100G weapons, this sort of application of power using high speed weapons is simply the opposite end of the scale to "stealth" ships. It is fairly obvious that all sides would attempt to have this card to play.

You are assuming that this would be an "asymmetric threat".Wars are expensive propositions. Even when given plenty of reason, most governments will do their best to achieve diplomatic solutions. This is especially true if the threat were indeed "existential". If a government has one such "existential threat" deployed, especially if it is a discovered stealth asset, how many others might it have deployed that are not known? Under such conditions, a declaration of war gives the "offender" no choice but to actually use all of its assets. Diplomatic solutions offer the "offended" government time to gather intelligence.

Going to war is not always a straight-cut solution a state can use, in my opinion. Most wars I know of were conducted on the premise of moral, religious or ideological reasons.

It would be interesting if 100G missiles and stealth ships existed in the same universe. There's be a risk that the missiles can be launched and reach their target before stealth ships are able to move into position despite already being in the target's orbit! This is because the solar thermal pulsed propulsion only gives millimeter-scale acceleration...

Sorry... I probably did not separate the two points well enough in my comment above. The first point is that both (all) sides would probably be developing the stealth tech to the best of their abilities. The second point was that even if it were asymmetric, it would unlikely result in an immediate declaration of war.

In Star Wars universe they had something like "stealth chemical engine", which gives me idea, probably just impossible, making rocket that uses some kind of reaction that doesn't create high temperatures, but expands a lot

Low temperatures means low exhaust velocity, so you'd need an enormous amount of propellant to get anywhere. You'd need high temperatures, high exhaust velocities, small propellant flow and very high expansion ratio to remain undetected.

Metastable materials like Metastable Helium or metallic hydrogen provide a "handwave" means of getting high thrust and ISP, but the practicality of these materials is open to question (to say the least). A ship with a metallic hydrogen "fuel" might need a spherical fuel tank made of diamond with walls about 1/3 the thickness of the diameter of the tank, for example.

On the subject of efficient, stealth propulsion systems - what about a matter beam rider?

That's a spacecraft that collects iron pellets, decelerating them with superconducting magnets and storing the energy in superconducting loops. It would fling the pellets the other way to dissipate that energy.

In theory it would be almost 100% efficient, and the whole spacecraft could be cooled to 3K, which means you can get very high currents in the right type of superconducting wire. Unfortunately, I think the pellets will heat up as they are decelerated, otherwise you could dump your waste heat in them. Also, the beam launcher station is going to be visible, but it might be possible to disguise whether it's active and where it's firing.

That will work by eliminating the waste heat from the propulsion system, but will lead to a very inefficient hydrogen steamer.

A 'cold' steamer, where liquid hydrogen is boiled and dumped overboard in its barely evaporated state, will remove 455kJ/kg of waste heat. Quite impressive. However, a 'hot' steamer, where hydrogen is heated to 3000K, then expanded to trade temperature for velocity, can remove up to 60MJ/kg, which is astonishing.

In other words, an external propulsion system will remove propulsion waste heat but cost the steamer a much higher heat capacity method of removing heat.

The pellets will heat up, but it is the result of the waste heat generated by the magnets inefficiently converting the magnetic field into kinetic energy. The waste heat is distributed between the magnets themselves and the pellet.

The solution to getting colder pellets is to have them be smaller and contain less kinetic energy per unit mass. However, if they get too small, the iron will be magnetically saturated and stop producing more work. I don't know how you could dump your waste heat in them without passing them through a heat exchanger. Iron is a poor heatsink, with only 0.46kJ/kg/K compared to hydrogen's 14kJ/kg/K (and hydrogen rises to 20kJ/kg/K as it gets hotter).

More real world reporting on metallic hydrogen. Scientists have finally reached the holy grail of creating metallic hydrogen and have assessed some of its properties. ITs possible use as rocket fuel is also discussed here: http://www.nextbigfuture.com/2016/11/co-discover-of-metallic-hydrogen-wrote.html.

So no stealth, but a much higher energy fuel than anything outside nuclear and nuclear fusion propulsion. Metallic hydrogen is supposed to be a superconductor as well, which provides some interesting possibilities for things like mag sails, magnetic shielding of spacecraft and other things.

Specifically, if the tank masses 100 times the mass of the contained metallic hydrogen, you can go ahead and divide all those performance numbers by 100. It would also be useless as a superconductor because the important factor there is current capacity per mass.

Thank frankly sounds like wishful thinking. A substance that requires extreme pressure diamond vices to create is going to be stable to 1 atm? Stranger things have been discovered but this seems pretty unlikely...

Personally, I don't doubt all that much that metallic hydrogen will reain solid at 1 atm. That said, saying that it (normally) remains solid at 1 atm is NOT the same thing as saying that it is necessarily STABLE at 1 atm. Also, regular hydrogen is solid at 1 atm... when cooled to below 14°K. Thus, the question here is, at what TEMPERATURE will it remain solid under 1 atm pressure?

Instead of using a giant Fresnel lens, what about using an on-board heat source? For example, using a high-temperature nuclear reactor may not heat hydrogen up as high as a solar-thermal engine, but it may be simpler and easier to use.

A realistic on-board heat source is much less efficient, but it would have to be used in the outer solar system.

Using something like a radio or microwave heater, the hydrogen plasma might reach even higher temperatures than with a solar thermal pulsed engine that is physically limited to about 6000K (the temperature of the sun's surface).

Yes, it does. This is the principal of the VASIMR engine. However, you need to be careful about shielding the heater, as RF and microwave emissions are MUCH easier to detect than IR, even at multiple AU ranges, and do NOT require a sensor to be colder than the source.

Would a spacecraft-mounted mass driver work as a low-emission propulsion system provided the re-mass is cooled prior to acceleration? I can think of a few possible issues, like the re-mass heating back up via induction or the waste heat generated by the driver's coils, but would that be enough to compromise stealth?

The overall heat signature includes the energy source, conversion systems (i.e. how you convert nuclear energy to electrical energy), and losses in each step of the system, including the coils of the mass driver.

Most systems are affected by the Carnot limit, so there is already a large heat signature in turning some form of energy into electricity (even PV cells have a "hot" and "cold" side), and no physical process is 100% efficient.

Of course, much of this is moot, since sensitive modern sensors (even with late 1990 or early '00's technology) are capable of observing differences in temperature measured in fractions of a K; you are gong to be spotted regardless of what you do. The real trick is either to mask you intentions or fake out your opponent so they are heading to the wrong place and at the wrong time to counter your move.

@Trevor Petersen:Thucydides touched on this. Even of the mass driver is extremely efficient, the power source is unlikely to be. Generating electricity on-board the spaceship will greatly increase the liquid hydrogen consumption compared to the design example in the blog post.

The mass driver's projectiles can be held inside an insulated bucket while it is accelerated. The bucket absorbs waste heat, but does not transmit it to the projectile and therefore only shooting out a cold projectile.

Compromising stealth is an issue tightly bound to the technological assumptions of the setting, and even then, there might be practical limits. Modern sensors are skirting the quantum limits of electronics technology, able to detect single photons and accumulating a picture over the course of months or years. However, they are design to look at stars. We don't know how effective they are when looking at much closer objects against more chaotic and warmer backgrounds. We don't know if 20K above absolute zero will trigger sensors in a single sweep, or if they can spot every heat source above 3K in a single snapshot of space...

Another issue that comes into play is that even if the thermal energy is absorbed, the mass driver coil is going to produce RF emissions. Such emissions are negligible for systems like VASIMR, but will be quite intense for mass drivers. The bad part is that RF emissions provide a significant photon flux, even at very low energy values. Detection is not so much determined by energy as it is determined by this photon flux.Also, while it is possible to shield such emissions (to some extent, at least), such shielding increases the thermal load that must be absorbed. In any case, the amount of RF power generated by mass driver coils makes such shielding difficult to accomplish.

Interesting concept... but the thing i can't make add up is how you are going to cope with the heat absorbed by your VantaBlack coating. With the lense deployed, the ship would fall into the shadow of the lense, but without it your stealth ship is going to absorb something close to 15MW of sunlight, which is going to ruin your endurance.

Even with the lense deployed, how are you going to stop the lense itself from shining in the sun? Are there any materials that will refract light with 0% reflection???

The heat absorbed by the VantaBlack is on the order of 4-10kW. Only the small plate in the front is exposed to sunlight. It is removed by boiling liquid hydrogen. Liquid hydrogen absorbs 455kJ/kg when going from a liquid to gaseous state.

The large lens focuses light into the furnace. If it can be made from diamond-like carbon, then it absorbs less than 2% of the incoming sunlight. With active cooling, it doesn't heat up, so doesn't emit anything. Light reflected back up from the furnace is stopped by an infrared filter.

If the material turns out to be too reflective, a Zone Plate can be used. It can be made from any opaque material, including Vantablack. The downside is that it absorbs between 50 and 75% of incoming sunlight. This hinders performance, but guarantees stealth.

I don't see how you can get the absorption that low on a cylindrical shape without using an external blocker of larger diameter. Even if you manage to keep the end of the cylinder perfectly aligned with the sun, the sun is of sufficiently larger diameter to still illuminate the sides. The absorption rate of VantaBlack is so high that the ridiculous low incident angle would be irrelevant, and it would still absorb over whatever % of the surface that sunlight struck.

I suppose you could get around this to an extent by tapering the cylinder, make the cold end narrow enough that the body is always in the shadow of the light inlet.

I get that you could shroud the built-in lense on the craft itself. you only need an annular sidewall a few meters long to hide reflections off the lense from anything not directly between the lense and the sun (though your IR filter doesn't make sense, these things work both ways, it'll stop as much IR from getting in as it does from getting out, which harms your engine power)

I really don't get what you mean about a zone-plate to obscure the deployable lense, though. The only way to stop it bouncing light off would be to stop light striking it in the first place.

The spaceship is designed to operate at Earth orbit (1AU) or beyond. At that distance, sunlight diverges at an angle of about 0.5-0.25 degrees according to different calculation's I've encountered.

I found this helpful image: http://www.powerfromthesun.net/Book/chapter02/Image48.jpg

To make it short, the cylinder's flanks are in the shadow of the front end.

I am unfamiliar with an 'annular sidewall'. Can you tell me about it?

The IR firlter works because the solar furnace and the sun's surface are a different temperatures, and therefore emit at different wavelengths. The furnace operates at 3000K temperature, which corresponds to far infrared. The sun's surface is at 6000K, with the majority of its energy in the optical band. An IR filter will therefore let most of the sun's energy through but block returning light.

The zone-plate is an alternative to conventional lens. Conventional lens reflect some sunlight, which compromises stealth. Zone plates can be made from any material, so if we use Vantablack, it will have near-zero reflectivity. Stealth is maintained, at the cost of 25% performance and 4x hydrogen consumption or more.

Possible point of interest: IIRC, there is research underway to attempt to incorporate VantaBlack into new gen solar cells, especially in combination with multilayer cells. This could bring cell efficiency up into the 80% or 90% range, or even higher. If you coat the vessel with such cells, this would reduce the thermal load considerably, and nicely supplement the power to (well shielded) RF/microwave heaters.

Atomic Rockets has a new post up on Space Forts, which seems to take most of the assumptions about heat, energy generation and protection and goes to the opposite extreme of encasing the battleship inside a massive construct or asteroid. (http://www.projectrho.com/public_html/rocket/planetaryattack.php#spaceforts)

The reasoning makes sense, you have a heat sink massing hundreds to millions of tons, even RBoDs and kinetic energy weapons moving at kilometres per second are simply absorbed in the mass of regolith and of course you can safely stash decades worth of MRE's inside (to the horror of the crew, no doubt).

As an actual space warship, this is the opposite of sleek greyhounds of the spacelanes or the HMS Surprise cutting through the waves. In fact, it isn'e even like the USS Iowa or IJN Yamato. In terms of performance, these things will be about as manoeuvrable as the USS Monitor of Civil War fame.

This also brings up a different paradigm of space warfare. If you are inclined to go into battle that way, your space warships will be about as easy to plot and see as the Aldrin Cycler. Maybe each polity makes an agreement to allow these cyclers to come by on a periodic basis as a visible means of showing the flag and ensuring diplomats have some actual constraints to operate under ("If the Mars issue isn't settled in 6 months, 25 days and 15 hours, their war cycler will enter Earth's Hill Sphere").

A bit simplistic, but a massive construct like that would be almost unstoppable, and could be either a warship in its own right, or act as a carrier for swarms of spaceships, drones and other equipment.

I think the endurance of such Space Forts is grossly overestimated. The majority of asteroids are porous rock barely held together. The few metal-rich rocks are not chunks of metal, but more like big boulders.

A boulder is a pretty bad replacement for armor. It will shatter upon impact, and lose a lot of its structural integrity when you start dumping your waste heat into it.

If you can plot the position of a space fort with accuracy, then you can reliably deliver a several dozen kilometer per second penetrating warhead. At 50km/s, a 10 ton block of iron contains 12.5 TJ of energy and only requires a mass ratio of 2 to be accelerated by any cheap ion drive. You can even save a few km/s of deltaV by impacting retrograde.

Kinetic attacks far outsrtip physical defenses, in my opinion.

The solution would be a million ton asteroid converted into a million tons of whipple shields. In that case, if you can the industrial capacity to build million ton objects, why not build a million tons of much more flexible warships?

While a passive space construct is certainly vulnerable to kinetic energy attack, the real point of the Space Forts is they have the size to carry countermeasures and fight back both as offensive platforms and with swarms of defensive weaponry.

Boring into a random asteroid is probably not the best starting point (although based on the date the opening part was written this composition of asteroids may not have been well known, but using a asteroid's worth of materials to build an actual space fort would provide the sort of construct the author seems to be talking about.

Approaching the space fort with fleets of missiles runs into the problem of attacking a target which could be potentially mounting several RBoD's, as it has the room and heat sinks for a large number of fusion generators and particle beam accelerators for the massive FEL's. Watching your force getting picked off at a light second (potentially farther, since the missiles don't have to manoeuvre to aim at the target) would be rather disheartening, to say the least.

As always, the real issue is what sorts of trade offs you are willing to make? A single million ton spacecraft that is bristling with weapons that no one will be able to overcome, or the ability to cover vast distances and bring more limited power to areas of interest?

In any "realistic" setting, space forts will most likely be orbiting important planets or moons (Mars has two convenient candidates). This complicates the issue for attackers, since they not only have to contend with a mobile space fleet, but a very hard fortress once they get into orbit as well. What is more important to the enemy commander. and how does he allocate his resources to deal with the two problems?

While there is a real issue of having an open cycle fission rocket near habitable planets and colonies the introduction of high thrust/high ISP engines would be a huge change in any interplanetary civilization. The other potential high thrust/high ISP drive is ORION pulse drive, which has issues of its own.

Moving very large payloads very quickly will be one of the drivers of economic growth and development (the other will be discovering low cost means of moving bulk cargos), and I'm pretty sure the "dismal science" will drive the rapid displacement of low thrust high ISP drives. Of course the limiting factor will be the availability of fissionables.

With Hydrogen Solar Stealthsips (HySS, for short) in action, we could justify the other popular sci-fi trope - space fighters!

Or more likely space anti-submarine crafts, because their main function would be to search, scan, inspect and identify.

How could we fight the enemy HySS's? Generally by providing extensive sensor coverage and inspecting all suspicious indirect signs that may indicate the presence of HySS or HySS-launched stealth munitions/sensor platforms. For that reason, we need small, cheap crafts, capable of providing sufficient coverage of orbits and trajectories to react in time on any confirmed HySS nearby the protected objects. Capital ships probably wouldn't be cost-effective for such role; the indications of HySS presence would probably be quite indistinguishable for just interference from small space debrees, small meteor bodies, ect. It would hardly be cost-effective to divert the thousand-ton space battleship, bristling with particle cannons and laser mirrors just to investigate any suspicious object - that may perfectly be just the fragment of some ancient rocket.

Also, if we use capital ships to search for HySS, we could face a problem; the HySS captain might decide that the sucsessfull destruction of our capital ship would justify the risk of HySS revelation.

So, for anti-HySS ships we need small, numerous and cheap units - cheap enough, that direct attack from HySS against them would NOT be cost-effective. I.e. fighters. Also, the HySS seems to be relatively "slow" ships, with pretty limited acceleration capabilities - and incapable of using direct-energy weaponry (which is most dangerous for small space crafts) due to power consumption and waste heat of direct-energy weaponry. So, they are, basically, perfect targets for small, maneuvrable attack crafts.

Of course, the question arise immediately "why not just use drones?", but there is the main thing: our anti-HySS crafts are mainly inspectors & killers. Their main goal is to investigate - and fast! - any suspicious indirect signs of HySS activity nearby our planets/asteroids. And they operate in conditions, when they must made a lot of assumptions and tactical decisions, based on indirect signs of possible HySS presence, and determine the rules of engagement by themselves. So, unless we have really sophisticated AI, we would probably stuck with pilots on anti-HySS crafts - of course, they would be supported with arrays of unmanned sensor platforms & drone crafts!

P.S. Must also point out, that there are other possibility - HySS-fighters. I.e. stealth small crafts, provided with vantablack outer coating, limited-endurance hydrogen heat sinks and "silent" engines for trajectory corrections. Such crafts could be transported by either HySS, or more conventional carrier and launched on ballistic trajectory toward enemy planet/asteroid/space colony. After silently running closing with the target, they drop the silent mode, activate their "hot" engines, and make sudden attack run, tearing apart the enemy capital ships & defense sattelites on their orbits.

Again, we could justify the pilots on them, because such fighters would be forced to operate under strict communication silense - and thus would need to make tactical decisions by themselves. They would need to chose targets, decide upon the attack scheme, avoid sensor platforms and enemy anti-HySS patrols...

So, it seems that HySS crafts could actually made a hard sci-fi space combat quite a bit more fun! :)

The great thing about HySS (I used 'hydrogen steamers' elsewhere) is that having a pilot onboard or not has very little impact of how much hydrogen you use per second to keep it cool.

I still think that we'll be sending 5-ship formations of 'inspector-hunters': one human-piloted craft hanging back, and 4 drones equipped with sensors trying to form a tetrahedron around the suspect signal to try to look at it from all angles. They won't be 100% drones, but not 100% human-piloted either.

Another interesting point is that this is a rare situation where decoys actually work. Mimicking the slightly-too-warm hydrogen emissions of a HySS shouldn't be very difficult.

Absolutlely! And yes, I completely agree - the HySS are great designs not only by themselves, but also because they make deep-space combat environment much more complex and sophisticated, making possible the large spectrum of tactical and strategical schemes and design variations, which otherwise would be not cost-effective.

I really like the idea of "inspector-hunters" semi-drone formation: this solution effectively combined the advantage of drone crafts (we don't need to put pilots on EVERY unit) with human decision-making capability.

//Another interesting point is that this is a rare situation where decoys actually work. Mimicking the slightly-too-warm hydrogen emissions of a HySS shouldn't be very difficult. //

Exactly. So, the tactical environment suddenly became cluttered enough, that we actually need to came close and look "what exactly it is" and we need humans to make decisions onboard.

Something to keep in mind: a stealth asset does not need to be in stealth mode at all times. Submarines going out to sea, and returning from missions, must pass through low water level bottlenecks forcing them to be VERY visible when passing between sea and harbour. Stealth aircraft are still currently flown from a limited number of well observed airbases. In both cases, such stealth assets are commonly observed throughout their mission... but such traces are just as commonly lost again.

Agreed, but we have the same problem as with submarines; while being excellent attackers, they are poorly suited to slug contest. The main problem of HySS - it could not carry powerful nuclear reactors or nuclear rocket engines; they would be the constant source of heat & revealing radiation. So, the HySS could not carry direct energy weapon - those systems are REALLY demanding - and could not do high-G maneuvres (hm, we probably could put a few old-fashioned LOX-hydrogen rocket on her, if we carry enough oxygen...)

That means that on the beam & slugs range, the HySS are toast. In missile exchange, the chances are better, but inability to use lasers and particle beams seriously impared the HySS ability to defend itself from the missiles.

So if the revealed HySS would not be able to surprise and destroy the enemy space dreadnought by the missile/HySS-fighter attack, the chances of HySS to survive the encouter is slim. The dreadnought could simply maneuvre better, could use beam weaponry and dreadnought armor would NOT absorb 99,999% of any radiation, aimed at her.

So, the HySS would probably never engage enemy ship by herself, unless she is forced to do this. She would fight by sending HySS-fighters or missile platforms, which would not reveal her position to the enemy. Of course, if the enemy managed to pinpoint the HySS exact trajectory, her crew would probably be forced to prepare the autocannons and short-range missiles - but the chances of sucsess would be as slim as for submarine to defeat the attacking destroyer by artillery. Possible, but unlikely.

I think the space version of sea bottlenecks is the departure and insertion burns spacecraft have to perform to reach the target. A HySS that wants to hide after passing through these bottlenecks must quickly start burning sideways with cold engines.

In fact, this might create a 'window of vulnerability' for HySS, just after the insertion burn, where fleets of inspector-hunters will be dispatched to catch it before it slips away...

@Алексей Широколава: Quite right. The way I see HySS attacking, though, is exclusively using missiles. Normally, in a battlefield dominated by beam weapons, missiles don't stand a chance of reaching their target. However, if they are launched at an unsuspecting energy from short range, they can bypass automated point defenses with minimal losses and detonate their nuclear warheads at point blank.

It becomes even more interesting if you use Casaba Howitzer warheads. Normally fragile and vulnerable to a laser strike, the Casaba Howitzers become devastating at short range and when fired without warning.

A more advanced concept is the kinetic stream. Thousands of tiny drones are released by the HySS, each equipped with a solar sail or undetectable ion drive. They take months to reach a very elliptic orbit. At the long end, they are millions of kilometers from the planet. At the short end, they are travelling at dozens of kilometers per second, either prograde or retrograde.

A HySS can deploy several of these streams at different inclinations. To trigger an attack, the drones use chemical thrusters to adjust their orbit by a tiny amount. They are like minefields. Deadly, undetectable, but have a very long reaction time.

In the insertion burn bottleneck, the HySS will probably be accompanied by escorts. For that matter, it is entirely possible that they will just be blended in with routine commercial and/or military traffic. Burns could also be organised in such a way as to minimise the ability of multiple platforms to conduct interferomtric observation, which would be a requirement for platforms to have ANY chance of determining burn directions, etc (there are a few other problems with the "you can tell everything from the plume" argument).Besides, submarines are just as (actually, MUCH more) vulnerable during the bottleneck passage... but they don't worry about it much since such manoeuvres are far removed from hostile platforms, and the overwhelming majority of such operations take place during relative peace time.

//It becomes even more interesting if you use Casaba Howitzer warheads. Normally fragile and vulnerable to a laser strike, the Casaba Howitzers become devastating at short range and when fired without warning.//

Basically, they are closest analogue to old, unguided naval torpedoes; utterly devastating, but only on a short range. Casaba-carrying missiles - basically the analogue of torpedo bombers; the idea is to bring short-range weaponry close to enemy onboard the smaller and cheaper unit.

Hot engines are indeedrather easy to detect... but perhaps not quite so easy as some would suggest. The first thing to keep in mind is that hot gases do not emit energy according to the same principals as blackbody sources... you do not get the same emissions for a given area. Second, I have seen, or suspected, analyses of exhaust plumes figure in the energy of propulsion sources to determine the range of detection... the problem here is that most of that rated energy is locked into the momentum of the exhaust particles. It is important to note that the only plume energy that is actually detectable is the thermal energy lost as electrons return to their lowest energy states within a molecule. While such thermal energies might be quite high for chemical thrusters, they are actually quite low for more energy efficient thrusters with high Isp values (the component going into exhaust velocity is undetectable... until the exhaust particle hits something). Third, detectability depends upon energy flux per unit of area. While a hot exhaust might be easily detectable from behind, where the exhaust cross section maintains its highest density, the cross section of a high Isp exhaust will spread that energy out over thousands of meters, meaning that the plume might very well be too thinned out to detect, even at relatively close ranges of millions of km. Finally, the energy of a plume is distributed in all directions, further diluting the observable energy flux reaching any given detector.

A tech question: could possibly the HySS concept be used for strategical movement of non-stealth ship?

I.e. one side wanted to move powerfull battleship from Earth to Mars and surprize the other side. They don't need her to stay stealthy after the transit, not even the whole transit. They just need her to be undetected long enough, so the opponent would not able to react and reinforce his Mars station forces in time.

Of course, ship that wasn't originally designed to be stealthy is much harder to hide. She probably would have much more emissions, and radar return as well. But we need her to stay stealthy only during part of the transit, after all.

The idea: the ship is accelerated on her hot rockets from Earth, seemingly toward some other point. During transit, she go stealthy (i.e. shut off her main power sources), and, using hydrongen heatsink and solar thermal rocket diverted her trajectory enough to be able to reach Mars. To confuse the opponent, the decoy - inflatable full-scale decoy with enough heat sources to imitate the cruising ship with her engines out - left on the old trajectory. The real ship is coasting a few month under stealth, then kick on her hot rocket and dash to Mars.

This depends a lot on the characteristics of the non-stealth ship. There are more factors than just propulsion system issues that interfere with, or otherwise affect, stealth. If you have a bright, shiny spacecraft, for example, this is going to reflect a lot of sunlight that will probably be quite easy to detect (especially if the vessel is fairly large). Likewise, if you have a large crew, lots of active machinery, and/or other heat sources, you are going to be radiating lots of easily detectable heat, unless you have lots of stealth insulation (note, having an effective heat sink is not sufficient... you need to prevent that heat from escaping, long enough for the heat sink to do its job, and that means insulation). Finally, if you have ANY active equipment that emits RF energy (I might be wrong, but this is likely to include most electronic items, or any item with a solenoid), it will probably be impossible to move the craft with any manner of stealth, even if the equipment were running on extremely low power. This is because the photon flux for RF emissions is incredibly large, even at microwatt levels. You might be able to shield such emissions, but this would likely require stealth planning before build... in other words, the ship will require some level of stealth to be built in.In short, it would be almost impossible to transfer a non-stealth ship in a stealthy manner, unless that ship were just dead cargo mass, with no crew and no active systems. That said, keep in mind that modern military vessels are designed to incorporate a number of stealth technologies, even though they are not intended for stealth missions. Thus, it would not be unthinkable for a non-stealth ship to have sufficient stealth tech built in to allow such manoeuvres.

As for the application involving decoys: such a scenario is unlikely. A decoy MIGHT work long enough in tactical situations where even a few seconds of uncertainty might be enough to break a target lock, IFF the decoy were deployed at exactly the right time. However, decoys generally do not work in such strategic applications even in today's environment. This is because analysis technology, combined with existing sensor quality, can generally distinguish important differences with very little effort. The only real decoy that might work for ANY strategic application would be to use another vessel of the same class, but one that does not carry the same crew nor armament... but this would be very expensive.

How far away is the enemy? Or, rather, how CLOSE?At 30°K, you are emitting well under 1/10 W of heat. Being extremely generous, this means that the photon flux will be (much) less than 1 photon / m^2 at ranges greater than 1 000 000 km.

Furthermore, it is impossible for any platform to detect waste heat emissions (IR frequencies and above) from sources colder than the detector platform. This is because the blackbody emissions from the platform itself will flood the detectors with exactly those frequencies being sought out.

Therefor, such a decoy would only be useful against platforms cooled below 30°K, AND closer than 1 000 000 km (or 10 000 000 km if the platform collector is greater than 100 m^2, etc). In other words, a waste of H2, except for tactical manoeuvres.

//That said, keep in mind that modern military vessels are designed to incorporate a number of stealth technologies, even though they are not intended for stealth missions. Thus, it would not be unthinkable for a non-stealth ship to have sufficient stealth tech built in to allow such manoeuvres.//

Hm. So you think that just average warship would be too hard to move undetected - but it is possible to create "semi-stealth" warship, which would have special adaptations to "silent running" (of course, at the cost of some combat capabilities)?

Pretty interesting idea... And quite useful.

// However, decoys generally do not work in such strategic applications even in today's environment. This is because analysis technology, combined with existing sensor quality, can generally distinguish important differences with very little effort.//

//The only real decoy that might work for ANY strategic application would be to use another vessel of the same class, but one that does not carry the same crew nor armament... but this would be very expensive.//

Hm, as I understood, the main problem is the engine. We could easily discriminate the decoy from real ship when the engine is burning - because of the mass flow, of the thermal emission of the exaust plume, spectral components, ect., ect., ect.

But this would not apply after the acceleration phase ended. During the inertial flight, the ship does not need to activate her engines for a long time.

Basically, I made an analogue between ICBM warheads and inflatable decoys, used onmissile buses. They disperced after the acceleration ended - and enemy could not discriminate them easily, because they have the same thermal emission and radar reflection. Of course, AFTER the warheads hit the atmosphere the lighter decoys would decelerate much faster... but then the enemy would have only few seconds to react)

The best example would be the absolute standard thermal flares and chaff equipped by every fighter jet.

>Hm, as I understood, the main problem is the engine. We could easily discriminate the decoy from real ship when the engine is burning - because of the mass flow, of the thermal emission of the exaust plume, spectral components, ect., ect., ect.

I actually think we three (you, Mikkel and me) all have the same idea with regards to decoys in space. They'd only work if the spaceship was designed to have minimal signature in the first place, and if it is not accelerating visibly.

https://www.nytimes.com/2016/10/13/world/europe/russia-decoy-The decoys you cite here are not exactly intended for strategic purposes, but large scale tactical measures intended to be combined with other techniques in subterfuge. Please refer the following article:

weapon.html?_r=0"They are intended for quick inflation and deflation: If they are left out for long periods, their airy nature becomes obvious to satellites, Ms. Oparina said, as they tend to blow around in the wind and swell and shrink in size."

More solid decoys are perhaps less likely to blow around, but they are just as prone to any in-depth observation. They will function as a means of confusing an enemy during combat engagements, but they will not stand up to strategic analysis.

Actually, there are some serious problems with the "exhaust plume analysis" theory. First, such analyses require a sufficent photon flux (not simply energy flux) across a large band of spectrum... such a photon flux is not even assured within a single frequency, especially at higher frequencies. Second, red/blue shifting will only indicate exhaust velocity components normal to the observer. Lateral velocity components can not be measured. Third, while spectrum distribution would theoretically determine the average exhaust temperature, such a technique is easily defeated through the use of multiple propellant/temperature components in the exhaust stream. For that matter, accurate resolution of the distribution is not assured. Fourth, while spectrum intensity should be proportional to mass, this technique is also easily defeated, as multiple exhaust products can not be independently analysed to determine how much each is contributing to the intensity. It is also quite inexact, as it is impossible to determine how much of the exhaust you are actually detecting. Fifth, the technique relies on being able to determine how fast the vessel itself is moving (and, in along which exact vector). The problem with this is that it requires very exact resolution, which in turn requires a long baseline (normally using interferometry), on the scale of tens of thousands of km, with accurate detection at both extremes of the baseline.

Instead, the problem is with the rapidly accelerating improvements in detection capabilities and automated analyses. At extreme distances, the decoys could work... but at such distances detection alone is difficult enough. Decoys would be relatively expensive overkill.

In most cases where interplanetary combat is regular enough to justify stealth designs, it is assumed that the defenders have positioned sensor platforms all around the target. Lateral velocity, I assume, would be deduced from observing the plume at different angles.

>multiple exhaust products can not be independently analysed to determine how much each is contributing to the intensity

While I agree with this, the injection of multiple propellants into the exhaust stream at different temperatures and velocities will only serve to confound the mass and/or the drive power output of the spacecraft. They don't seem effective in hiding the presence of the spacecraft entirely.

>turn requires a long baseline (normally using interferometry), on the scale of tens of thousands of km,

I think this can be achieved by a single platform observing the target at different points in its orbit... unless burns are sudden bursts of acceleration that last a couple of seconds!

>At extreme distances, the decoys could work

I hope we are not confusing things here. Drones may work well enough to allow a stealth ship to slip away, but can only replicate the emissions of a 'hot' ship with difficulty, and it is nearly impossible to decoy a hot, accelerating ship.

"In most cases where interplanetary combat is regular enough to justify stealth designs, it is assumed that the defenders have positioned sensor platforms all around the target."First, it is incorrect to say that regularity of combat justifies stealth designs. In fact, the condition would be almost exactly the opposite: regularity of combat would justify the increase of sensor platforms, which would disuade the construction of "classical" stealth craft (such as the F-117), and instead favour the construction of non-stealth craft employing stealth techniques only for purposes of making target locks a little more difficult to acquire in tactical situations.Second, trying to surround a target is not quite so simple when dealing with orbital mechanics. Platforms do not remain in relative position, which requires an abundance of platforms (or a lot of propellant on a fairly constant basis)... most difficult would be maintaining platforms able to resolve the "z" vector component, as this MUST be out of plane with the target. You also need a sufficient number to maintain a minimal count f platforms within detection range, which might (or might not) be possible with the big torchships, but might not not be as easy with simple solid core NTRs.That said...Yes. If you have at least three platforms that are able to accurately determine the spectrum shift, then you should be able to extrapolate the primary vector of the exhaust plume. However, this is where other issues come into play. For example, even if you can determine the average vector, you can not accurately determine the amount of exhaust divergence, nor distribution of mass across the divergent angles. Another problem is that you would not be able to determine off-centre burns (which would decrease the efficiency of a burn considerably, but would not be problematic if you have sufficient propellant).

"the injection of multiple propellants into the exhaust stream at different temperatures and velocities will only serve to confound the mass and/or the drive power output of the spacecraft. They don't seem effective in hiding the presence of the spacecraft entirely. "Quite correct. This is not, in itself, a stealth consideration. Instead, the purpose is to confuse exhaust analysis. By doing so, it will be possible to defeat projections of ship vector (it will not be able to accurately project where the craft will go between burns). This, then, applies as a non-stealth segment of an otherwise stealth mission. Sort of like the subs that are easily seen until they enter deep sea and go silent long enough to lose any followers.

"I think (interferometry) can be achieved by a single platform observing the target at different points in its orbit... unless burns are sudden bursts of acceleration that last a couple of seconds!"Sorry, no. Yes, such techniques are frequently used for sythetic aperature analyses. However, single source synthetic aperature only works with non-moving targets. Any movement disrupts the interferometric measurements. This is frustrated not only by acceleration and/or significant linear movement, but simply wobbling or rolling the craft can prevent accurate interferometric analyses. Also, for the baselines that we are talking about, it would take hours (at least) for a single platform to attain the necessary baseline endpoint.

"I hope we are not confusing things here."No, I don't think so. Under the conditions that such a decoy would work (insufficient resolution for analyses, insufficient signal for detection, etc), there would be no need for the decoy.

>regularity of combat would justify the increase of sensor platforms, which would disuade the construction of "classical" stealth craft

One could argue that slipping through such an 'alert' battlefield only makes stealth more worthwhile.

> instead favour the construction of non-stealth craft employing stealth techniques only for purposes of making target locks a little more difficult to acquire in tactical situations.

I think comparisons to modern stealth hurt this reasoning. Today, a stealth aircraft has to be designed from scratch to defeat Radar and IR scanners. A spaceship could theoretically 'bolt on' a supercooled shell, stick a bunch of hydrogen tanks to the hull and become a stealth craft.

Also, the HySS concept could be taken to absolute physical limits. A helium loop cooled to 3K by a heat pump connected to a hydrogen steamer would be undetectable even to the most advanced sensors. A modern aircraft will eventually be detected by sufficiently powerful and numerous radars. The analogy breaks down at that point.

Also, I believe that stealth in a realistic setting blurs the line between tactical and strategic, as it operates over months if not years. Wars can be won or lost before a HySS is forced to reveal itself.

>Second, trying to surround a target is not quite so simple when dealing with orbital mechanics.

I understand. However, the difficulty is relative to the pace of interplanetary travel. If everyone uses week-long milligee burns and spends months in interplanetary space, then you only need 3 sensor platforms to be in suitable positions every week or so. This can be achieved by placing multiple satellites in the same off-plane orbits, intersecting points regularly, synchronized, to take 3-axis snapshots of space.

> Instead, the purpose is to confuse exhaust analysis

This would be very useful during the departure burn, as we wouldn't know what the enemy is sending out. However, I believe it is pretty pointless from that point onwards, as any burn trying to confound its thrust/vector/Isp would be the target of powerful active detection efforts. Orbital defense platforms might have several gigawatts of power available for active detection, giving them extreme range against spacecraft with non-Vantablack hulls. There's even an induced fluorescence method using bursts of X-rays which I know no method of beating. It'd turn any physical hull into a strobe light. Therefore, any sort of detection would need to be avoided.

"One could argue that slipping through such an 'alert' battlefield only makes stealth more worthwhile. "Stealth is worthwhile, yes. It will therefore be a constant subject of research. However, IFF there is such an over-abundance of detectors (a condition which I actually doubt), stealth is less likely to be used, due to the cost and the low probability of success.

"I think comparisons to modern stealth hurt this reasoning. Today, a stealth aircraft has to be designed from scratch to defeat Radar and IR scanners."There has actually been a step back in the most recent "stealth" designs in aircraft. The current application of both aerial and naval stealth support my argument. Also, the current generation of stealth is no more "designed from scratch" than any other (non-stealth) vessel or aircraft.

"A spaceship could theoretically 'bolt on' a supercooled shell, stick a bunch of hydrogen tanks to the hull and become a stealth craft."Not quite so simple, but not far off, either. The propulsion system needs to be stealthed from the onset. Also, supercooled stealth has a very limited lifespan.

"A helium loop cooled to 3K by a heat pump connected to a hydrogen steamer would be undetectable even to the most advanced sensors"Probably,yes. That has been my argument. Although, the heat pumps will burn through the heat sink a lot more quickly.

"A modern aircraft will eventually be detected by sufficiently powerful and numerous radars. The analogy breaks down at that point."Which analogy are you refering to? Modern stealth aircraft are detectable NOW. The problem is, the return signal is so low that radar software will ignore the return. This has been my point in saying repeatedly, 'stealth is less about not being detected, and more about not being noticed.'

"I believe that stealth in a realistic setting blurs the line between tactical and strategic, as it operates over months if not years. "it is not the timescale that determines the difference. However, the point I was trying to make is that IFF the unlikely situation developes that there is an over-abundance of detection platforms, as posed by this scenario, stealth would no longer be able to endure the long time spans, but only extremely short spans during combat engagement. AGAIN, I consider such a situation highly unlikely.

"If everyone uses week-long milligee burns and spends months in interplanetary space, then you only need 3 sensor platforms to be in suitable positions every week or so."Let's say you want to box Mars in a surveillance zone, using three axis analysis. Mars has an average orbit of 228 million km. Assuming a detection range of 10 million km (coinciding with 8kW of assumed detectable exhaust plume energy from a VASIMR running at 100kW), you probably want this value to be your baseline radius (spacing between platforms) for field overlap. The good news is that platforms in nearby orbits will tend to remain in position more on the order of months than days or weeks. The bad news is that once they drift out of position, they will remain out of position on the order of years. This means that you will need a number of platforms, regularly spaced, in each orbit.So. How many?Well, you can actually have two platforms in the same orbit as Mars, one leading and one tailing. These will remain in position virtually permanently. The bad news is, in order to avoid gravitational influence, and the intervening position of Mars, their detection ranges probably won't be able to provide the required observational overlap. You could use one or the other, but not both.After that, you will want an in-plane orbital platform inside Mars' orbit, at about 220 million km from the sun. But this is where drift comes into play. If you want to assure overlap with both Mars-orbit platforms, then you will want to have successive platforms coming into play at about 5 million km intervals. You don't need them all at once, but eventually you will need 276 such platforms in orbit (you can make do with fewer, but you will alrready have some gaps in coverage).Next, you want another in-lane platform just outside Mars' orbit, at 235 million km. This poses the same problem, so you will eventually need another 295 platforms.Now you get to the off-plane orbital platforms. I have good news, bad news, and good news.The good news is that you don't need hundreds of platforms in each orbit. Instead, you should be able to get by with two or three platforms in an interior orbit, and another to or three in an exterior orbit. We will go with two, and accept some loss of overlap. You can get away with this because you can select orbits with the same period as that of Mars... they just have to be more elliptical.The bad news is that off-plane orbits don't shift position to follow on-plane planets (etc). You need platforms in as many different orbits as you would otherwise need platforms within an orbit. This means 276 sets of orbits, minimum (unless if you want large lapses in overlapping coverage).The good news? Well, since the orbits are elliptical, there is nothing preventing you from having sets of orbits that swap the positions of the interior and exterior pairs at opposite extremes. Instead of 4 platforms per orbit, you now only need two.The total comes to 1125 platforms (you will have some loss of overlapping coverage on the far side, but that should not be too worrisome). To summarize, this allows for three platforms to provide overlapping, 3D coverage regardless of where the target is in martian orbit.This is just for Mars. If you want to box in other planets, you will need many more per planet. Again, you could make do with less, but that will open up large windows for stealth craft to slip through.

"Orbital defense platforms might have several gigawatts of power available for active detection, giving them extreme range against spacecraft with non-Vantablack hulls."The problem is that, with active scanning, you run into the inverse FOURTH power distance rule. At a single km range, a GW flux is reduced to a single W. At 1000 km, the GW flux is reduced to a nW flux return signal.Even without VantaBlack, though, active scanners are very easily defeated at range. Given the problem with return signal, even partial signal absorbtion reduces the return well below detectable levels. But you don't even have to rely on absorbtion. Flat panel reflection works quite well to reduce return signal as well, especially for lasers or very high frequency EM emissions.

"There's even an induced fluorescence method using bursts of X-rays which I know no method of beating."There are a few problems with this.First, induced x-ray flourescence requires a higher frequency source (higher frequency x-rays or gamma rays). These have quite poor photon flux to begin with.Second, the maximum photon flux, for lowest frequency x-rays, is about 3.3*10^17 photons/W which reduces detection range for the return signal considerably.Third, according to one paper I found, a laser beam with a concentrated energy flux of 60 GW/CENTImetre^2, ON THE TARGET, yields a return of 3*10^-16 J, again at target. At this level of conversion, there is no hope at using this technique at ranges above a few metres.

Returning to the notion of using decoys: it has occurred to me that we all appear to be making the assumption that the decoy has to have the same observable characteristics as the actual vessel... which is nonsence, unless the observer already knows what they are looking for. What decoy has to do, in terms of stealth aplications, is to draw attention from the vessel.Actual vessels and craft, for instance, can serve as very useful decoys for other, completely different, constructs. A series of explosions could serve as a decoy, if they have interesting enough characteristics.

Neither do chaff flares, still the prefered decoy for modern military aircraft. Neither did fuel oil dumped into the sea, often with a few spare parts and some trash... rather useful decoys in WWII for subs attempting to escape destroyers with depth charges.They just have to look real enough to seam worth the time to investigate.

Of course, the problem with an explosion is that there might not actually be anyone looking when you want them to look. A better idea would be something along the lines of FIREFLY's "Cry Baby". An emergency message from just far enough away to require a significant amount of resources to look for. Somewhere where a rescue attempt from Earth might arrive in time, but where assets even in the same orbit might not.You don't even have to mimic propulsion systems (etc), because presumably they are disabled.

You could store and launch stealth streamers from hangars that are presumed to be other objects in an active space economy. While with great logistical expenditure stealth systems serve as secret transports for goods, systems and ships(similar to drop tanks or conformal fuel tanks) between established space ports. In both cases stationary systems in these ports could be used to launch stealth streamers at high speeds. These stationary systems do often serve a non-military infrastructure at the same time, requiring a degree of clever disguise for the launches that are not meant to be observed. It might be helpful if such launches are carried out with unmanned cargo, which can be manned at their destination.As soon as a stealth streamer enters combat, the stealth component is lost, so having secret stealth transports of ships is sueful. If the ships are launched for combat, due to the vastness of space and the impeding loss of stealth in space, I presume it sufficient if they have lowered observeability to counter combat targeting(but not stealth). The cooling of for low IR is only useful as a short term measure in combination with a decoy to redirect active targeting of an incoming strike. Warfleets might use visible active measures of fog that due not deny their existance, but make obtaining for correct military threat evaluation and targeting solutions difficult.

That's a decent plan, but the necessary limitations on the transport operation would limit it to some sort of special-ops or first strike.

What I mean is - the hidden-in-plain-sight transport craft would have to disguise their cargo and maintain operational security both up to the release of the stealth ships they are transporting, but also some time after the release, otherwise observers would become very suspicious and start actively searching the area.

If that is to be achieved, the transport ships would have to act like they normally would. In space, this means that they would follow minimum energy trajectories that cost the least for commercial operations. They would have to submit a flight plan and continuously broadcast their position so as to prevent collisions and accidents that involve megaton yields.

This means that even if perfect operational stealth is achieved, the warships would have to give up all positional advantages by being released along a predictable, well-frequented and heavily scrutinized trajectory, passing right through the heaviest defenses and the densest sensor networks of their target.

An analogy would be if you modified an oil tanker to conceal a submarine, but you could only release the submarine somewhere along the length of the Panama canal.

They also give up all tactical flexibility, as such an operation cannot be launched at any moment's notice. Whatever forces you send would have to submit to the travel times of civilian craft, and expose themselves to the risk of their opponent refusing to allow civilian craft the opportunity to approach them by declining flight plans that pass nearby. Deviations from these flight plans will never pass off as 'mistakes' or 'errors', as the trajectory change is costly in time and energy, and if you do not have control over your craft, who is to say that that same loss of control won't send it careering into a space station or populated orbit and kill people down the line?

To move away from this trajectory, the stealth ships would need a decent propulsion system.

This begs the question though: why not skip the transport ship and just move the stealth ship into a favourable position under their own propulsion?

I have discussed in 'Expansion-Cooled Curved-Nozzle rocket' how even propulsion systems such as Nuclear Thermal rockets can remain stealthy and yet provide thrust that is comparable to non-stealthy designs, minus a handicap.

If a stealth steamer enters combat... well, that is debatable.'Combat' might involve dropping missiles, something that does not require an expenditure of energy. 'Combat' might be the fact that the stealth steamer is a carrier for a non-stealth combat ship. The combat ship is detected, but the stealth ship remains hidden. The stealth steamer can use the same liquid hydrogen heat sink to absorb the waste heat of weapons fire for a very short period of time... although being able to fire at an unsuspecting enemy with even a single laser pulse or coilgun projectile might be enough to take them out.

Even if the stealth ship uses non-stealthy weapons, such as powerful lasers that it cannot conceal, it might always elect to stop shooting, flush the radiators with liquid hydrogen and retract them, and return to its extreme-low-observability mode. The further away it is from its opponents when it does this, the easier it is to slip away with thrust from stealthy 'cold' propulsion systems. It is a not a strict all-or-nothing stealth system like in sci-fantasy, where 'camouflage' can be switched on or off. It is up to the enemy ships to keep track of a stealth-capable ship through other means, such as continuously lighting it up with a fluorescence-inducing laser or degrading its low-temperature envelope by burning it off or spraying it with metallic paint.

The problem with 'countering combat targeting' is that when lasers are available, being able to see something is strictly the same thing as being able to shoot at it, and even if you have the general position of a target, you can employ active detection systems (radar, lidar) that can eventually pin-point its location based on the signal returns.

Fog might be effective in concealing IR emissions and even hindering radar returns. The mass of the fog cloud necessary to achieve this effect is incredible however, and it increases exponentially if you want it to survive any reasonable amount of time under a high-powered laser trying to burn it away. And, that fog cloud is an expendable resource that doesn't follow the ship when you move out of the way of any blind-fired kinetics and missiles.